Two-way data communicating method and system

ABSTRACT

A plurality of stations (S), for instance measuring probes, preferably comprising respective microprocessors (KL), are connected successively to a common two-wire line (SL). Supply voltages and synchronization signals are distributed through this line (SL) from a central unit (CU), and at the same time a full duplex communication between single stations (S) and the central unit (CU) is enabled.

The present invention relates to a method and a means for two-way datacommunication between a central unit and a number of stations, and thecommunication takes place via a single two-wire line, with the centralunit and the stations places successively along the line.

On example of a more specified system, is a system for collectingmeasurement values from a number of stations having sensors or measuringprobes which monitor electrical/physical parameters. However, a stationlay also be a control unit which in addition to the monitoring function,also controls parameters of a system attached to the station. Inaddition to the transmission of information from one single station tothe central unit, it is also possible to put the system in a mode whereinformation from one station can be transmitted to another station.

A more specific example of utilization of the invention, is monitoringof voltage and temperature in lead accumulators in a submarine orelectrical emergency batteries in power plants, telecom or similarinstallations. It is a common feature in such uses that the accumulatorcells are placed together as a battery of cells in a room which isclassified as explosive.

However, as a principle, the present invention represents a quitegeneral system for, using many stations attached to the same two-wireline, both transmitting and receiving signals between a common centralunit, hereafter also referred to as "group interface", and singlestations connected to the two-wire line, possibly also signals betweenthe stations. In important embodiments care has been taken to providegalvanic separation, i.e. no direct DC connection, between the variousunits and between the units and the line. In further importantembodiments, supply voltage and timing signals are also provided to thesingle stations via the two-wire line.

In accordance with the invention a method is provided for two-way datacommunication such as stated in the appended patent claim 1. Favourableembodiments of the method in accordance with the invention are definedprecisely in the appended method claims 2-13. The invention alsocomprises a system for two-way data communication, and the system isdefined precisely in the appended patent claim 14. Further favourableembodiments of the system are indicated in the appended patent claims15-20. The invention shall now be illuminated further with reference toembodiment examples, and while referring to the appended drawings, where

FIG. 1 shows a system in accordance with the invention in an embodimentwith galvanic separation between the elements, and where addressing ofthe single stations takes place by signalling from the central unit,

FIG. 2 shows a system where a central unit and stations are connected toa high-voltage power line and uses this line as a signal transmissionmedium,

FIG. 3 shows a system which is rather similar to the system appearing inFIG. 1, however with a third, special line which is used in connectionwith addressing of the stations, and

FIG. 4 shows a system where addressing is made by means of a device forapplying a variable DC voltage to the two-wire line.

It is first referred to FIG. 1. The system central unit CU, alsoreferred to as the common group interface, comprises an oscillator O, avoltage divider network R1, R2, and an attached PET switch F1. Theswitch F1 is controlled from the processor μP. The oscillator O is alsoconnected to the processor μP which uses the signal from the oscillatorin connection with timing/synchronizing.

D1 and D2 are diodes facing in opposite directions and connected inparallel, the diode D2 being series connected with a resistor RA. A1 isan amplifier (comparator) for measuring current through RA. In theembodiment appearing in the figure, galvanic separation between thecentral unit CU and the two-wire line SL is provided by means of atransformer TO. Instead of a transformer one may possibly use twocapacitors as a galvanic separation means.

One of the stations connected to the two-wire SL appears in the righthand side of the figure, with reference letter S. The station comprisesin the embodiment shown, where galvanic separation is included, atransformer TN, where a resistor R3 is series connected to the primaryside Of the transformer. As appears, the station is typically ameasuring probe where measurement voltages are received via high-ohmicresistors RS by a control logic circuit KL, which possibly, however notnecessarily, is a microprocessor. The control logic circuit KL receivessupply voltages U1, U2, U3 from a network which follows the transformerTN. The network comprises diodes and capacitors called D3/C3, D4/C4,D5/C5, respectively. Fences in the shown embodiment the operating powerfor the station originates in the oscillator O in the group interface CUand is delivered via the line SL. The voltage U2 can be switched off/onby means of the FET switch F2 which is controlled from the control logiccircuit KL, in order to obtain an absolute minimum current consumptionin the total system.

Further, the oscillator O in the group interface circuit CU provides thesystem clock frequency, i.e. not only for the processor in the centralunit, but also for the stations. The clock frequency can be extracted ina simple manner in each station, and this is shown in the figure by thewire named CL, which leads from the secondary side of the transformer TNto the control logic circuit KL. A phase shift due to TO, TN and theseries resistor R3 can be compensated for in a simple manner ifnecessary.

Several stations can be connected to the same common two-wire line. Themaximum number is determined by the required transmission rate,signal/noise ratio and by general requirements regarding a possible EXcertification, i.e. if the stations are located in dangerous areas.

Important further elements for back signalling from the station to thecentral unit, are elements F3, R4 and D6, i.e. respectively a FETswitch, a series resistor and a diode. This branch, where the switch iscontrolled from control logic circuit KL, is connected in parallel withthe voltage supply branches comprising C3/D3, C4/D4 and C5/D5.

Correspondingly important for the function is the comparator K, which isarranged to detect a voltage change on the secondary side of transformerTN, with origin in a voltage change in the central unit CU when there ismade controlled use of switch F1. The output of comparator K isconnected to the control logic circuit KL, and one input thereof isconnected to the secondary side of the transformer TN, while the otherinput is connected to a reference voltage.

The manner of operation for the system is as follows: If a signal is tobe transmitted from the central unit or group interface CU in principleto all stations, this is executed by modulating the oscillator signalusing the switch F1. It is to be noted at first that the oscillatorsignal is not necessarily a normal sine oscillation, but it mustgenerally have a periodic character and comprise two opposite polaritieswithin the period. Thus, a typical example of such a signal is a sineoscillation. In principle the signal modulation is made by e.g. changingthe amplitude of the positive half period between two, or possibly more,values. The duration of the change can be chosen equal to all of or partof the half period. In the solution shown in FIG. 1, two values areused, the FET switch F1 being switched on and off, thereby causing thatthe voltage amplitude in the positive half period is at a "high" levelwhen the switch is open, and "low" when the switch is closed. Thus, adigital signal can be transmitted using the positive half periods. (Thenegative half periods light equally well be chosen for such an outgoingsignal from the central unit CU.)

The signal amplitude change will also be present on the secondary sideof the transformer TN in each station. The voltage change is detected inthe comparator K, which passes digital signals on, corresponding to thesignal. entered in switch F1. There are certain restrictions to how many"low" amplitudes that can be transmitted successively in a simplemanner, however this poses no problem in the embodiment in question.

Moreover, it can be favourable, in order to achieve minimal changes ofthe DC voltage component of the signal, to let the amplitude be thesame, but with opposite polarity in two half periods. This does notchange the fundamental function of the modulation.

Thus, in the embodiment of interest here, the digital signal out fromthe group interface CU will contain an address of a certain station, andwhen this address is detected in the addressed station, that particularstation, and only that station, wil enter into activity. In this casethe station has a pre-programmed identity number in internal storage,i.e. the address, and this identity number is recognized whentransmitted from group interface CU. The system then is of a type whereonly one single station will be in activity at a time. (This can also berealized in other manners, and we shall return to this later.)

When the station S of interest has been activated, it is intended totransmit signals back via the same wire line SL. This occurs in the caseshown by making the control logic circuit KL control opening/closing ofthe FET switch F3, and notably only in the negative half periods foroscillator O (that is when the outgoing signal from the group interfaceCU utilizes the positive half periods). When the switch F3 isopened/closed, the electrical current in the two-wire line SL varies,and this current change will then be detected in the same negative halfperiods by investigating the current through resistor RA in the centralunit. The duration of the current change can be selected equal to orpart of the half period. The current changes are digitized in theamplifier Al which supplies a signal to the central unit processor. Alsoin this case there are restrictions regarding how many half periods witha large current draw that can be transmitted in succession, but thiswill not represent a problem for the embodiment in question.

It may be favourable to let the current change be the same, but ofopposite polarity in the two half periods, in order to achieve minimalchanges of the DC voltage component of the signal. This does not changethe principal function of the modulation. In other words, the system canbe arranged in such a manner that modulation of one bit into thetwo-wire line from the central unit lasts one complete period, at thesame time as the modulation in return from the station, regarding onebit, also lasts for one complete period. But the information is inprinciple attached to the voltage amplitude in the "outgoing" signal (inthis case starting in a positive half period), and to the currentamplitude (starting in a negative half period) in the return signal fromthe station. Such a methodology will also provide a rather practicalsolution technically.

In FIG. 2 a somewhat special solution is shown, where the two-wire lineSL to be used, is a cable in a high voltage power distribution network,e.g. a 220 volt AC current distribution network. By connecting thetransformers TO and TN (the same reference letters/numerals are usedregarding corresponding elements from one figure to the next) to thecable SL, via a coupling member KE1, KE2 respectively, it will bepossible to use this live cable as a communication line. Moreover, itwill be possible to take out operation power from the cable high voltage(not shown), while the data carrying, high frequency voltage from theoscillator O in the group interface CU is used for providingsynchronizing signals for timing the functions of the stations.

In FIG. 3 one will note the following difference in relation to what isshown in FIG. 1: An additional and special line has been drawn from thecentral unit processor AP to the control logic circuit in the firststation. Thus, in this case, S is the first of a number of stations. Theline is galvanically separated by means of a capacitor C1.Correspondingly, a line has been drawn from the control logic circuit KLin the first staton, to the corresponding control logic in the nextstation, and galvanic separation has been provided also in this case, bymeans of a capacitor C6. Apart from a resistor R5 inserted in thestation, all remaining elements, identical to what is shown in FIG. 1.

The manner of operation is in this case that the first station shallalways respond first. When this first station has finished, it providesan enabling signal to the next station via the line containing capacitorC6. Hence, this type of sequential scanning/interrogation is differentfrom what was stated in the mention of FIG. 1, but for the rest thesignalling form between the central unit CU and the station S is the,same, i.e. via the two-wire line SL and by means of modulation of theamplitude of the respective period parts, respectively voltage andcurrent.

In FIG. 4 appears one further form of addressing, and it should be notedthat FIG. 4 also is rather similar to FIG. 1, except for the followingspecial features: In FIG. 4 the station S is shown to be a measuringprobe which in this case measures the voltage of an accumulator cell CN.Wires from the cell terminals enter the probe control logic circuit KLvia high ohmic resistors RS, as previously mentioned.

Thus, in the case shown, the particular station S which appears in thedrawing, is connected to one particular accumulator cell, which has acertain potential on one terminal. A next station or measuring probe Swill be connected to the next accumulator cell in the row, and willtherefore be attached to the (higher) potential oe the next cell, etc.The fact that one certain measuring probe or station S is attached to acertain potential, is utilized in the addressing system appearing here:In the central unit CU there is arranged a DC voltage generator UA, witha negative side connected to safety ground in the figure. Similarly, thenegative terminal of the first accumulator cell is connected to safetyground (in principle, only a colon potential is required). The variableDC voltage from generator UA is connected to one of the wires in thetwo-wire line SL, and thus determines the line potential. It is thenpossible to compare the line potential to the potential attached to therespective measuring probe S, and this is made by means of a comparatorKA which has been introduced in the measuring probe. A series resistorRB is connected in front of one input of comparator KA, while the otherinput is connected via resistor RS to the accumulator potentialcorresponding to measuring probe S, i.e. to the negative terminal ofaccumulator cell CN. A difference signal passes from the comparator KAvia line SA to the control logic circuit KL. In circuit XL is then madean evaluation regarding whether the difference between the twopotentials lies within a predetermined range, and if so is the case, thestation/measuring probe is activated to re-transmit measuring values tothe central unit CU. It is thus possible to determine which measuringprobe S shall be activated, by regulating the DC voltage provided by thegenerator UA in the central unit CU, controlled by the central unitprocessor μP.

Of course it is not in principle necessary that this is a case ofmeasuring accumulator cells, but an attached, measurable and specificpotential for each respective measuring probe or station S is necessaryto utilize this manner of addressing.

Thus, a "window" is selected for the control logic circuit, and thiscircuit will start data transmission when the DC voltage of the two-wireline is in a certain range between negative and positive potential foraccumulator cell CN. By increasing the voltage from generator UAgradually from 0 volt and up to the total voltage of the completeaccumulator series, each respective measuring probe (which is connectedto a respective cell) will receive an activation signal (as well as ade-activation signal). Each measuring probe will therefore only beactivated when the DC voltage potential on the two-wire line SLcorresponds to the potential to which the measuring probe is connected.

It should be noted that the resistance of resistor RB can be made verylarge, for example 10 Mohm, so that the small DC current passing fromthe two-wire line to the measuring probe will not represent any safetyrisk, while assuring that an accumulator cell area is a typicalexplosive area.

As mentioned above and previously, an important use of the presentinvention is just in acquiring measurement data from accumulator cellsplaced in a collection as a battery of cells in a room classified asexplosive. For example in a submarine there say be as many as 200 singlecells connected in series, while in emergency power batteries there willoften be 24 cells connected in series. Hence, potential differences willappear between the single probes, as high as 48 volt for emergency powerbatteries, while submarine batteries may reach 4-500 volt DC.

In traditional sensor solutions these potential differences will createlarge practical and safety-related problems. Further, there will oftenbe a large amount of wiring to be arranged to each respective cell whentraditional measuring methodology is used.

There are sensor systems where many measuring probes can be connected toa common data acquiring line. In some of these systems a single probewill take its supply voltage from the cell it is placed together with.In such connections there are two problems. If the measuring probe drawscurrent from the cell in which or together with which it is arranged, itwill not be possible to disconnect the measuring probe completely toprovide a "dead" state. Being able to achieve such a "dead" state isimportant in a number of applications, particularly in a submarine,where it is desirable to disconnect all unnecessary electronics tominimize emission of radio signals from the vessel.

It further turns out that a number of problems arise when satisfying therequirements for self-safety. In total it has turned out that the sensorsystems developed for such purposes, are far more costly than what usersare prepared to pay.

The present invention solves in a practical and above all veryinexpensive manner the above mentioned problems. The invention can beimplemented at a cost of perhaps only 10-20% of the present systems.

As further safety related considerations in connection with the abovestatements, the transformer TN which is used at the input of astation/measuring probe, must have an inductance low enough to maintainthe requirements for a design which is self-safe. This entails that anyDC voltage component in the communication line disappears. For acommunication line in its simplest form, without other logic circuitsthan what has been indicated at the start, possible error detections mayarise for special code combinations. In other words: When the comparatorA1 in the group interface CU detects a signal in (e.g.) the negativehalf period, this may be due to either that the previous positive halfperiod was low, or that a measuring probe/station has return signalledwith a larger current draw. This phenomenon is due to the fact thattransformer TN has an inductance that is too low. However, this problemcan be solved in a simple manner, since the processor μP in the groupinterface CU actually has knowledge about the amplitude of the lastpositive half period.

Thus, it is clearly possible to run a full duplex communication on thetwo-wire line SL, without putting restrictions on what signals arecoded. However, microprocessor treatment of the signals will be requiredon both sides. The processor on each side in the communication hasitself knowledge about the signal last transmitted therefrom. Therebythe changing DC voltage states of the signal can be controlled.

By having the resistor R3 inserted between the station transformer TNand the two-wire line, current limiting is obtained. By choosing acorrect combination of values of R3, TN and the synchronizationfrequency, the system will on one hand be able to operate in a quitecorrect tanner, while when a possible error occurs, it will have amaximum current draw which is limited by the resistor R3. when an erroroccurs in the measuring probe, this current draw can be madesufficiently low so that the measuring probe may receive an Exi approvalwithout costly safety devices etc. at the input.

By making the control logic circuit KL of the measuring probe. whichcircuit may very well be a microprocessor, receive measuring values viathe series resistors RS which preferably may have a resistance of about2 Mohm, the measuring probe will also in this regard be able to satisfythe leakage requirement for an Exia approval. In the constructiondescribed in FIG. 4, the measuring probe will almost be a "passive"element.

So far the stations have only been examplified as measuring probes.However, a station may also provide outgoing control signals, e.g. forcontrolling a process. These control signals say e.g. pass toflip-flops, digital/analog converters etc., which are controlled fromthe control logic circuit of the station, which preferably then is amicroprocessor. This microcontroller/microprocessor receives itsinstructions from the group interface CU according to the generalcommunication system which has already been described.

It is further to be noted that if the signals generated by theoscillator O are sine curves, it will be possible to reduce the emittedelectromagnetic waves to a minimum. If the change in the amplitude ofthe sine signal is made (i.e, opening/losing of switches P1 and F3) whenthe sine curve passes a zero, the amount of superharmonic frequencieswill be reduced to a minimum, and this will reduce emanation strongly.

Finally, a system option for transmitting a message directly from onestation (A) to another station (B) shall be described: In a system whereall stations are connected continuously and where each station has itsown, unique address number, such a communication can be made in thefollowing manner:

Station A transmits a message to the group interface CU and establishesthis message as a "mirror" in a certain number of periods of the clocksignal. In this period of time the group interface CU will receive thesignal entering from the outside in one half period, and retransmit itto the two-wire line in the next half period. Of course, this signalmust simultaneously be coded (through a number of "bits") with anaddressee, so that the correct station recognises its address.

Morover, it lust be mentioned that synchronization of the central unitCU and the station S with which communication is desirable, can beestablished in different manners. Techniques known in the field ofordinary serial data communication can be used. Alternatively, a doubleamplitude change (either from the central unit CU or from the station S)can then be used as a start signal. The point of time when a station isaddressed, can also be detected in the central unit through an increasedcurrent draw. One has also envisioned the possibility of transmitting astart bit, either from the central unit or from a station, which startbit has a very low amplitude.

What is claimed is:
 1. A method for two-way data communication via anelectrical two-wire line between a processor controlled central unit anda number of stations each having a logic control unit, where saidcentral unit generates a variable and periodic electrical voltage signalbetween the two wires, each period of the voltage signal comprising afirst and a second period part, the amplitude of the voltage signalduring the first and second period parts having opposite polarities,wherein the method is characterized in that said central unit varies thevoltage amplitude of a series of first period parts to send aninformation segment out on the two-wire line, while a station varies thecurrent amplitude correspondingly for a series of second period parts bycontrolled change of the input impedance of the station, wherein thestation uses the variable voltage from the central unit as a powersupply voltage and a synchronizing signal for execution and timing ofthe function of the station, and whereby a return information segmentfrom the station is detected by said central unit by measuring currentchanges caused by the impedance changes of the station.
 2. The method ofclaim 1, wherein the method is further characterized in that voltageamplitude and current amplitude are changed in a pre-determined timewindow in each period part in question.
 3. The method of claim 1,wherein the method is further characterized in that said central unitgenerates the same voltage amplitude form in both first and secondperiod parts, and that the impedance change of the station iscorrespondingly made in full period time units.
 4. The method of claim 1where said two-wire line constitutes a cable in a high voltage powerdistribution network, and wherein the method is further characterized inthat the central unit draws electrical operation power from the cablehigh voltage.
 5. A method for two-way data communication via anelectrical two-wire line between a processor controlled central unit anda number of stations each having a logic control unit, where saidcentral unit generates a variable and periodic electrical voltage signalbetween the two wires, each period of the voltage signal comprising afirst and a second period part, the amplitude of the voltage signalduring the first and second period parts having opposite polarities,wherein the method is characterized in that said central unit varies thevoltage amplitude of a series of first period parts to send aninformation segment out on the two-wire line, while a station varies thecurrent amplitude correspondingly for a series of second period parts bycontrolled change of the input impedance of the station, whereby areturn information segment from the station is detected by said centralunit by measuring current changes caused by the impedance changes of thestation, and wherein the station comprises a microprocessor controlledmeasuring probe for at least one physical parameter, the measuring probemicroprocessor holding the station address in store and delivering aresponse containing measuring data when information transmitted fromsaid central unit contains the address of the station.
 6. A method fortwo-way data communication via an electrical two-wire line between aprocessor controlled central unit and a number of stations each having alogic control unit, where said central unit generates a variable andperiodic electrical voltage signal between the two wires, each period ofthe voltage signal comprising a first and a second period part, theamplitude of the voltage signal during the first and second period partshaving opposite polarities, wherein the method is characterized in thatsaid central unit varies the voltage amplitude of a series of firstperiod parts to send an information segment out on the two-wire line,while a station varies the current amplitude correspondingly for aseries of second period parts by controlled change of the inputimpedance of the station, whereby a return information segment from thestation is detected by said central unit by measuring current changescaused by the impedance changes of the station, and wherein each stationcomprises a measuring probe for at least one physical parameter and aplurality of measuring probes are interrogated successively in such amanner that a first measuring probe receives an activating signal fromthe central unit on a special activating signal input via a specialline, communicates during a pre-determined time period with said centralunit, and transmits at expiry of the time period a second activatingsignal to a corresponding, special activating signal input of a nextmeasuring probe via a special line.
 7. A method for two-way datacommunication via an electrical two-wire line between a processorcontrolled central unit and a number of stations each having a logiccontrol unit, where said central unit generates a variable and periodicelectrical voltage signal between the two wires, each period of thevoltage signal comprising a first and a second period part, theamplitude of the voltage signal during the first and second period partshaving opposite polarities, wherein the method is characterized in thatsaid central unit varies the voltage amplitude of a series of firstperiod parts to send an information segment out on the two-wire line,while a station varies the current amplitude correspondingly for aseries of second period parts by controlled change of the inputimpedance of the station, whereby a return information segment from thestation is detected by said central unit by measuring current chancescaused by the impedance changes of the station, and wherein the stationcomprises a measuring probe for at least one physical parameter, whereeach station is connected to a certain electrical potential, and whereinthe method is further characterized in that the DC voltage potential ofthe two-wire line is regulated separately by means of a voltagegenerator which is controlled by said central unit and is connectedbetween one of the wires and a reference point,that a comparator in alogic control unit of the measuring probe determines the differencebetween the DC voltage of the two-wire line and the certain potentialconnected to the measuring probe, and that the measuring probe isactivated for measuring and communication when said difference lies in apre-determined value range.
 8. A method for two-way data communicationvia an electrical two-wire line between a processor controlled centralunit and a number of stations each having a logic control unit, wheresaid central unit generates a variable and periodic electrical voltagesignal between the two wires, each period of the voltage signalcomprising a first and a second period part, the amplitude of thevoltage signal during the first and second period parts having oppositepolarities, wherein the method is characterized in that said centralunit varies the voltage amplitude of a series of first period parts tosend an information segment out on the two-wire line, while a stationvaries the current amplitude correspondingly for a series of secondperiod parts by controlled change of the input impedance of the station,whereby a return information segment from the station is detected bysaid central unit by measuring current changes caused by the impedancechanges of the station, and wherein each station comprises amicroprocessor controlled measuring probe for at least one physicalparameter, and where the measuring probes are possibly situated in anexplosive or fire-prone environment, and wherein the method is furthercharacterized in thatthe variable voltage from the central unit isconnected to the two-wire line through a galvanic separation means, eachstation/measuring probe receives the variable voltage through a galvanicseparation element, the current between the two wires in the two-wireline being limited by a resistor connected in series with the primaryside of the galvanic separation element, and a plurality of high-ohmicseries resistors are used on the measuring inputs of the measuringprobes to limit measuring current values for safety reasons.
 9. Themethod of claim 1 wherein the method is further characterized in thatthe periodic voltage signal generated by said central unit issubstantially sinusoidal, and that amplitude variation is made both bythe central unit and a station with a start in the zeros of thesinusoid.
 10. A method for two-way data communication via an electricaltwo-wire line between a processor controlled central unit and a numberof stations each having a logic control unit, where said central unitgenerates a variable and periodic electrical voltage signal between thetwo wires, each period of the voltage signal comprising a first and asecond period part, the amplitude of the voltage signal during the firstand second period parts having opposite polarities, wherein the methodis characterized in that said central unit varies the voltage amplitudeof a series of first period parts to send an information segment out onthe two-wire line, while a station varies the current amplitudecorrespondingly for a series of second period parts by controlled changeof the input impedance of the station, whereby a return informationsegment from the station is detected by said central unit by measuringcurrent changes caused by the impedance changes of the station, andwhere the station comprises a process control unit comprising amicrocontroller, and wherein the method is further characterized in thatthe control unit microcontroller delivers control signals to processequipment on the basis of data received from the central unit, andtransmits in return process measuring data acquired by attached sensors.11. A method for two-way data communication via an electrical two-wireline between a processor controlled central unit and a number ofstations each having a logic control unit, where said central unitgenerates a variable and periodic electrical voltage signal between thetwo wires, each period of the voltage signal comprising a first and asecond period part, the amplitude of the voltage signal during the firstand second period parts having opposite polarities, wherein the methodis characterized in that said central unit varies the voltage amplitudeof a series of first period parts to send an information segment out onthe two-wire line, while a station varies the current amplitudecorrespondingly for a series of second period parts by controlled changeof the input impedance of the station, whereby a return informationsegment from the station is detected by said central unit by measuringcurrent changes caused by the impedance changes of the station, andwherein said central unit, when receiving a specially coded data messagefrom a station, enters a reflection operation mode wherein the followingmodulated second period parts are repeated directly by the central unitfor transmission in each successive first period part.
 12. The method ofclaim 11, wherein the method is further characterized in that saidreflection operation mode is provided with a duration determined in thecoded data message, and that the data communication thereafter takingplace from said station under this duration, also comprises an addresscode which puts one certain of the remaining stations in a responsemode.
 13. A system for two-way data communication via an electricaltwo-wire line between a processor controlled central unit and a numberof stations each having a logic control unit, optionally also betweenthe stations, where said central unit is adapted to generate a variableand periodic electrical voltage signal between the two wires, eachperiod of the electrical voltage signal comprising a first and a secondperiod part, the amplitude of the voltage signal during the first andsecond period parts having opposite polarities, wherein the system ischaracterized in thatsaid central unit comprises a circuit for processorcontrolled amplitude variation of the voltage signal between the twowires, output data from said central unit being represented by thetwo-wire voltage signal in a series of first period parts, said centralunit further comprising a detection circuit for electrical current inthe two-wire line for a current direction corresponding to the polarityof the second period parts, said detection circuit delivering ameasuring signal to the processor, each station has an impedance circuitfor varying the input impedance of the station as viewed from thetwo-wire line, said impedance circuit being controlled by the logiccontrol unit of the station, input data to said central unit from astation being represented by the two-wire line current in a series ofsecond period parts, each station comprises at least one network forproviding supply voltages for the functions of the station from thetwo-wire line voltage, and that the logic control unit of the station issynchronized by means of the two-wire line voltage.
 14. The system ofclaim 13, where said two-wire line comprises a cable in a high voltagepower distribution network, and the system is further characterized inthat said central unit comprises a network for taking a supply voltagefrom the cable high voltage.
 15. A system for two-way data communicationvia an electrical two-wire line between a processor controlled centralunit and a number of stations each having a logic control unit,optionally also between the stations, where said central unit is adaptedto generate a variable and periodic electrical voltage signal betweenthe two wires, each period of the electrical voltage signal comprising afirst and a second period part, the amplitude of the voltage signalduring the first and second period parts having opposite polarities,wherein the system is characterized in thatsaid central unit comprises acircuit for processor controlled amplitude variation of the voltagesignal between the two wires, output data from said central unit beingrepresented by the two-wire voltage signal in a series of first periodparts, said central unit further comprising a detection circuit forelectrical current in the two-wire line for a current directioncorresponding to the polarity of the second period parts, said detectioncircuit delivering a measuring signal to the processor, each station hasan impedance circuit for varying the input impedance of the station asviewed from the two-wire line, said impedance circuit being controlledby the logic control unit of the station, input data to said centralunit from a station being represented by the two-wire line current in aseries of second period parts, each station comprises a measuring probefor at least one physical parameter, and each measuring probe comprisesa microprocessor which has a station address in store, and which isadapted to submit a response containing measuring data when informationtransmitted from said central unit contains the station address.
 16. Asystem for two-way data communication via an electrical two-wire linebetween a processor controlled central unit and a number of stationseach having a logic control unit, optionally also between the stations,where said central unit is adapted to generate a variable and periodicelectrical voltage signal between the two wires, each period of theelectrical voltage signal comprising a first and a second period part,the amplitude of the voltage signal during the first and second periodparts having opposite polarities, wherein the system is characterized inthatsaid central unit comprises a circuit for processor controlledamplitude variation of the voltage signal between the two wires, outputdata from said central unit being represented by the two-wire voltagesignal in a series of first period parts, said central unit furthercomprising a detection circuit for electrical current in the two-wireline for a current direction corresponding to the polarity of the secondperiod parts, said detection circuit delivering a measuring signal tothe processor, each station has an impedance circuit for varying theinput impedance of the station as viewed from the two-wire line, saidimpedance circuit being controlled by the logic control unit of thestation, input data to said central unit from a station beingrepresented by the two-wire line current in a series of second periodparts, each station comprises a plurality of measuring probes for atleast one physical parameter, and between two successive measuringprobes there is connected a special activating line for transferring anactivating signal from one measuring probe to the next when apredetermined period of time for transmitting information to the centralunit has expired, said activating signal activating a correspondingtransfer of information for the next measuring probe.
 17. A system fortwo-way data communication via an electrical two-wire line between aprocessor controlled central unit and a number of stations each having alogic control unit, optionally also between the stations, where saidcentral unit is adapted to generate a variable and periodic electricalvoltage signal between the two wires, each period of the electricalvoltage signal comprising a first and a second period part, theamplitude of the voltage signal during the first and second period partshaving opposite polarities, wherein the system is characterized inthatsaid central unit comprises a circuit for processor controlledamplitude variation of the voltage signal between the two wires, outputdata from said central unit being represented by the two-wire voltagesignal in a series of first period parts, said central unit furthercomprising a detection circuit for electrical current in the two-wireline for a current direction corresponding to the polarity of the secondperiod parts, said detection circuit delivering a measuring signal tothe processor, each station has an impedance circuit for varying theinput impedance of the station as viewed from the two-wire line, saidimpedance circuit being controlled by the logic control unit of thestation, input data to said central unit from a station beingrepresented by the two-wire line current in a series of second periodparts, each station comprises a measuring probe for at least onephysical parameter, and each station is connected to a certainelectrical potential, a DC voltage generator which is controlled fromsaid central unit, is connected between one of the wires in the two-wireline and a reference point, for instance ground, the logic control unitof the measuring probe comprises a comparator which receives thetwo-wire line DC voltage and the certain connected potential of themeasuring probe in order to determine the difference therebetween, andthe measuring probe is adapted to be activated for measurement andcommunication when said difference lies in a pre-determined value range.18. A system for two-way data communication via an electrical two-wireline between a processor controlled central unit and a number ofstations each having a logic control unit, optionally also between thestations, where said central unit is adapted to generate a variable andperiodic electrical voltage signal between the two wires, each period ofthe electrical voltage signal comprising a first and a second periodpart, the amplitude of the voltage signal during the first and secondperiod parts having opposite polarities, wherein the method ischaracterized in thatsaid central unit comprises a circuit for processorcontrolled amplitude variation of the voltage signal between the twowires, output data from said central unit being represented by thetwo-wire voltage signal in a series of first period parts, said centralunit further comprising a detection circuit for electrical current inthe two-wire line for a current direction corresponding to the polarityof the second period parts, said detection circuit delivering ameasuring signal to the processor, each station has an impedance circuitfor varying the input impedance of the station as viewed from thetwo-wire line, said impedance circuit being controlled by the logiccontrol unit of the station, input data to said central unit from astation being represented by the two-wire line current in a series ofsecond period parts, a station comprises a measuring probe for at leastone physical parameter, and the measuring probes optionally are situatedin an explosive or fireprone environment, a galvanic separation means,is inserted as a transfer member between the central unit and thetwo-wire line, between said two-wire line and each measuring probe thereis arranged a galvanic separation element, where also a current limitingresistor is connected in series with the primary side of said galvanicseparation element, and that the measuring inputs of the measuringprobes are equipped with high-ohmic series resistors to limit themeasuring current values for safety reasons.